The invention relates to bidirectional switching power supply (SPS), particularly for power supply systems requiring a conversion of an AC or DC input voltage into single AC or multiple DC output voltages. A power transformer is used if line isolation is necessary. For instance, the SPS can be employed in uninterruptible power supply (UPS) system for converting a low battery voltage into 120 V/60 Hz or 220 V/50 Hz, and for charging the battery.
Conventional SPSs with single AC or multiple DC outputs provide either crudely stabilized DC voltages or employ regulator for each DC output. An isolation transformer is employed whether or not line isolation is required. One or two large capacitors are employed to store voltages prior to the transformation. After rectification, two other large capacitors store bipolar voltages which are converted into the AC output signal having the desired waveform. Pulse width modulation (PWM) is used which results in numerous problems including high circuit complexity, stability problems and significantly higher effective switching frequency. Numerous interference suppressors and protection circuits are inevitable.
Power transformers may be the worst components used in electronics, their employment demands specific considerations. For instance, the transformers demonstrate huge EMI and RFI, poor efficiency, leakage inductances, parasitic resistances, resonant elements, frequency limitations, flux symmetry problems, nonlinearities and saturation, bidirectional feedback, inability to provide DC signal, large size and weight, high cost. The imbalance in operating flux level is reduced by a series coupled capacitor which must sustain AC voltage and high currents. High frequency switching is essential in order to minimize size of the transformer.
UPS systems provide AC power during commercial power fluctuations and interruptions. An UPS comprises a charger, battery and inverter. The charger rectifies the line voltage and charges the battery. The inverter converts the battery voltage and provides the AC power. A continuous UPS system is coupled in series with line. The charger must be able to deliver enough power to drive the inverter at full load, maintain the charge on the battery and handle momentary overloads. A forward transfer UPS system is coupled in parallel with line and delivers AC power only during the power failure. A bidirectional transfer switch connects the critical load either to line or inverter output. The battery voltage is usually very low.
Half bridge and full bridge converters are commonly used in high power SPSs. Full bridge converter is preferred if very high output power is required. However, full bridge converter demands two times more power semiconductor devices than half bridge converter. This magnifies often most essential problems associated with mechanical arrangement, controlling, driving and protection of the power devices. In contrast, half bridge converter demands semiconductor devices with double voltage ratings and a pair of supply voltages having opposite polarities in reference to one output terminal. Specific measures must be taken in the converter itself in order to obtain adequate levels of supply currents. However, prices of the devices having double voltage ratings are often only slightly higher. Moreover, the SPS according to the present invention provides a pair of voltages which can have even values and opposite polarities.